A magnet is an object that has a magnetic force and attracts iron powder. The term ‘permanent magnet’ refers to a magnet having a strong magnetic force, which is manufactured for industrial use. Generally, the permanent magnet is commonly called a magnet.
Iron pieces disposed around a magnet are attracted by the magnet. The space affected by the magnetic force is called a magnetic field. In other words, it can be said that the magnet generates the magnetic field. The shape of the magnetic field may be ascertained using a magnetic field pattern obtained with iron powder. When a thick piece of white paper is placed on the magnet and then the iron powder is uniformly scattered on the paper, the magnetic field pattern becomes visible. When a small magnetic needle is placed on the pattern, it orients itself in a direction depending on lines of magnetic force. The lines of magnetic force from the N-pole of the magnet run to the S-pole of the magnet.
The force between the two poles follows Coulomb's law, in which the force is inversely proportional to the square of the distance between the two poles, and is proportional to the intensity of the magnetic poles. The product of the intensity of the magnetic poles and the distance between the two poles is defined as a “magnetic moment.” The magnetic poles are essentially formed of a pair of N and S-poles having the same intensity, and thus the magnetic moment is considered as a substantial physical quantity rather than as the intensity of the magnetic poles. The magnetic moment is represented as a vector directed from the S-pole to the N-pole. When the force between two magnetic moments is calculated, it is inversely proportional to the fourth power of distance. For this reason, the attractive force between two magnets increases as the magnets become closer to each other and rapidly decreases as the magnets become more distant from each other.
Magnetization is performed through a change in the shapes, arrangement and orientation of magnetic domains. Once an object, having a structure in which such characteristics are almost unchanged, is magnetized, it is not restored to its original state even after a magnetic field is reduced to 0, and thus a magnetic moment remains. Such an object having a large residual magnetization is a permanent magnet.
The term ‘magnetic flux’ refers to an amount obtained by performing integration on a sectional area perpendicular to the direction of magnetic flux density or magnetic induction. The unit of magnetic flux is Maxwells, Mx, in the CGS unit system and is Webers, Wb, in the MKS unit system or the SI unit system. When a magnetic flux passing though coil varies with time, a voltage that is proportional to the variation rate thereof is induced between both ends of the coil (Faraday' electromagnetic induction law). The direction of the voltage is the same as the direction of the disturbance of the variation in the magnetic field induced by the current. This is called ‘Lenz's law.’ The magnetic flux is generated by current passing through a permanent magnet or coil.
There are various types of sensors corresponding to methods of detecting a magnetic field, and the most widely known sensor is a hall sensor. The hall sensor operates in such a way that, when a magnetic field is applied in a direction perpendicular to a semiconductor device (hall device) while current flows through the electrodes of the semiconductor device (hall device), an electric potential is generated to be perpendicular to the directions of the current and the magnetic field.
As the simplest device for measuring distance, there is a device using a permanent magnet and a sensor for detecting magnetic flux. The device measures magnetic flux density varying according to variation in the distance from the permanent magnet and measures the distance based on the voltage potential generated from the sensor.
However, the magnetic flux density generated by the permanent magnet is not linearly formed according to the distance, so that a program or electronic circuitry for compensating for nonlinearity is provided in the device in order to use the device as a sensor for effectively measuring distance. Accordingly, the device can function as a device for accurately measuring distance only when this condition is satisfied. Furthermore, in order to compensate for the distribution of nonlinear magnetic flux density depending on distance, which is generated by a single magnet, extensive research into a structure in which a linear magnetic flux density can be achieved by combining various types of magnets with a plurality of magnets has been conducted.
Recently, various types of non-contact distance measurement devices, each of which, in a linear range or in an angular range, detects the absolute location of a body and measures displacement that is linear and forms an angle, have been developed.
There are various schemes for detecting measurement location in a non-contact manner. Although a device using a sliding resistor or a potentiometer is the most representative, the reliability thereof is not satisfactory. Although an optical positioner has an optical sensor for reading an optical range, such as a slit range, the structure thereof is more complicated. In addition, although there is a magnetic range in which a range recorded on a magnetic medium is read by a magnetic sensor, the structure is also complicated, and thus the absolute location cannot be detected.
That is, only the distance between two arbitrary points can be measured. The present invention provides a magnet having a linear magnetic flux density, which can detect the absolute location of a body to be detected, and has a very simple structure, a long measurement range and high reliability.
A conventional device is configured such that a magnetic sensor 14 measures distance while moving relative to a permanent magnet 12 in the direction of the polar axis of the magnet 12. The schematic perspective view of the conventional device for detecting a location (hereinafter refers to as a ‘location detector’) is shown in FIG. 1. In the drawing, the permanent magnet 12 is arranged opposite the magnetic sensor 14. In this case, the distance L between the permanent magnet 12 and the magnetic sensor 14 may vary. That is, relative movement between the two members is permitted. The magnetic sensor 14 moves relative to the permanent magnet 12 in the direction of the polar axis of the magnet 12. A magnetic field emitted from the permanent magnet 12 is sensed by a sensitive magnetic sensing device that is included in the magnetic sensor 14. The distance L is indicated and detected by an indication signal output from the magnetic sensor 14. However, in the above-described construction, the effective distance, which is a highly linear characteristic curve, is very short.
Furthermore, as shown in FIG. 2, a closed magnetic circuit 28 includes a U-shaped yoke 22, a permanent magnet 24, and a magnetic sensor 26 having a magnetic resistance sensing element, such as a Baber pole-type element. The magnetic sensor 26 moves relative to the permanent magnet 24 in a direction perpendicular to the polar axis of the magnet 24. However, in the above-described construction, it is also difficult to obtain an accurate linear characteristic curve.